The problem with your idea is that not everyone uses suppliers to sell on an eBay or an Amazon alone or even at all. Many use to sell on their own websites where there is no competition and of course less cost, meaning the potential for margin is greater. Some also buy in wholesale and sell at market stalls and so on, so if we were to reject suppliers based on that criteria, then that would effect those members unfairly.
Even if we asked suppliers if they did sell direct to the public so we could inform members of that upfront, many would trade under a different name and simply tell us they don't and of course there is no way we would be able to verify that for members.
So while it's a frustrating part of e-commerce in today's market, it's not something we can counter through a verification process unfortunately. All we can do is verify the trader is legitimate for the trading safety of members, we can't even really verify them on prices because of course that relates back to how members are using the suppliers.
While one supplier may be too expensive to compete on eBay for whatever reason, that same supplier might be perfect for a member selling through their own site where they have the ability to make more margin due to lower operating costs.
E-commerce isn't really a one size fits all kind of deal, what isn't right for one person, can be perfect for another and that's why it's important for us to base our verification process on safety alone, and then members can work on who is best for themselves from there.